In the manufacture of web based products, such as paper, textiles and certain plastics, it is sometimes desirable to apply a coating to the surface of the web such as a starch coating or other polymer coating. Having a suitable formulation, such coatings provide improved gloss, slickness, color, printing detail, or brilliance to the particular web being manufactured. Unfortunately, such coatings have a tendency to accumulate on the web manufacturing equipment, periodically requiring removal. As will be appreciated, any downtime necessary for the removal of accumulated coating material can result in substantial cost to the manufacturer. Consequently, there is a need in the manufacture of web based products for methods ad apparatus for controlling the flow of coating material in order to minimize downtime required to clean or remove such material from the manufacturing equipment.
Coatings are typically applied by a coater head which is moved into a position approximate the web which in turn is generally carried by or tensioned against a roll or drum. The distance the coater head is spaced from the web generally determines the thickness of the coating. More particularly, such coater heads usually are provided with a blade, the leading edge of which is oriented at a certain angle relative to the direction of movement of the web and resting on the web, applying a certain loading. The loading of the blade leading edge determines in most instances the thickness of the coating being applied.
Generally, in order to assure a flow of coating material onto the web, behind the leading edge of the blade, a pool or reservoir chamber of pressurized coating material is formed between the coating head and the web with the blade forming one wall thereof. End dams are typically positioned at either end of the reservoir chamber, abutting the blade and forming the end walls of the reservoir. The back and bottom walls of the reservoir are formed from other components of the head and in some instances may be integral therewith. Unfortunately, the effectiveness of the prior end dams in maintaining the coating material within the reservoir chamber has been unacceptable.
In order to understand the effectiveness of such prior end dams, consider first their construction, for example, an end dam used in the coater head of a short dwell blade coater. This coater head is provided with a finger-like projection having a width generally equal to the width of the coater head and which extends into the reservoir. The end dam, typically made of felt or a flexible synthetic material, is provided with a corresponding groove so that the end dam can be positioned at any point along such projection. The end dam also includes an end wall which defines one end of the reservoir chamber. As is known, the spacing of a pair of end dams along the projection determines the width of the coating to be applied to the web.
The end dam effectiveness problem at first stemmed from the formation of the end dam from felt material. The felt would absorb the coating material and change shape to an extent that the blade was moved out of position which produced a non-uniform coating on the web. In an attempt to resolve this problem, end dams constructed from foam rubber were mounted in the coater. However, since it is undesirable to have this type of material contact the web during the coating operation, it was necessary to space by a small amount the surface of the rubber end dam from the moving web. Coating material leaked through the space between the web and the top of the end dam wall. Since the coating material is pressurized, a small portion will flow continuously through this narrow opening and collect on the web manufacturing equipment. After awhile so much of the coating material builds up on the equipment that the manufacturing process must be halted in order to remove the material, resulting in unwanted downtime.